In the existing semiconductor fabrication processes, dummy wafers are used in furnace processes to block or stratify gas flows and to uniform the temperature distribution within a furnace so as to make reactant gases carried by the gas flows evenly heated and evenly contact with the wafers processed and hence the reactant gases react chemically or physically with the wafers processed to deposit or grow a thin film on the wafers processed.
Furnace wafers used in a low-pressure chemical vapor deposition (LPCVD) furnace process for polysilicon deposition generally have a silicon-silicon nitride structure, as shown in FIG. 1, consisting of a silicon substrate 1 and a silicon nitride layer 2 with a thickness of 50 nm to 200 nm over the silicon substrate 1. Such structure enables the underlying silicon substrate 1 to be protected by the silicon nitride layer 2 from being eroded by an acidic solution during a cleaning process performed to the dummy wafers after they are retrieved, thus extending life spans of the dummy wafers.
Nevertheless, during the deposition of gate polysilicon of a metal-oxide-semiconductor (MOS) device, process wafers disposed in the furnace always have a silicon dioxide layer formed thereon, as a gate polysilicon layer is generally formed on a silicon dioxide dielectric layer. As a result, the process wafers and the dummy wafers have different materials on their surfaces with silicon dioxide on surface of the process wafers and silicon nitride on surface of the dummy wafers.
As different materials have different absorption rates against reactant gases introduced in the furnace process, it will lead to different deposition rates of reactant gases deposited on surface of the different materials. Therefore, after a certain amount of the reactant gases are introduced in, a greater amount of gas will be consumed on surface of the material having a higher deposition rate while a smaller amount of gas will be consumed on surface of the material having a lower deposition rate, which will affect the thickness of the film deposited over surface of the process wafers.
Therefore, in a polysilicon deposition process, when conventional dummy wafers with a silicon nitride layer formed thereon are disposed in the same furnace with process wafers, polysilicon layers with a thickness deviating from the desired value will be deposited on process wafers due to an absorption rate difference between silicon nitride and silicon dioxide against the reactant gases.
Thus, there is a need to improve the structure of the conventional dummy wafers to avoid their adverse effect on thicknesses of deposit layers of process wafers in a furnace process.